t/t6022: a new test for renaming merge.

This adds a couple of tests to cover the following renaming
merge cases:

 - one side renames and the other side does not, with and without
   content conflicts.

 - both side rename to the same path, with and without content
   conflicts.

The test setup also prepares a case in which both side rename to
different destination, but currently the code collapses these
destination paths and removes the original path, which may be
wrong.  The outcome of this case is not checked by the tests in
this round.

Signed-off-by: Junio C Hamano <junkio@cox.net>